Phase I study of abiraterone acetate (AA) in patients (pts) with estrogen receptor– (ER) or androgen receptor (AR) –positive advanced breast carcinoma resistant to standard endocrine therapies.

2525 Background: Many advanced ER+ breast cancer pts show intrinsic resistance to endocrine treatment with the remainder acquiring resistance. Androgenic steroids upstream of aromatase can drive steroid receptor signaling critical to tumour growth. Evidence also exists for an ERα-/AR+ subset of breast cancers transcriptionally similar to ERα+ disease. We hypothesized that AA, a cytochrome (CYP) 17 inhibitor that irreversibly inhibits androgen and estrogen synthesis, would have anti-tumour activity in ER+ or ER-/AR+ pts. Methods: Post-menopausal women with ER+ or ER-/AR+ advanced breast cancer resistant to >2 lines of hormone therapies were treated in 6-pt cohorts with AA at doses between 250 and 2000mg daily. Results: A total of 25 pts were treated in the phase I trial with 2 pts ongoing on study. Median time on treatment was 1.8 mths (range 0.7 – 11.6 mths). There were no dose limiting toxicities. The majority of adverse events (AEs) were Common Toxicity Criteria grade 1 or 2: fatigue, nausea, anorexia, ...